Magnetic resonance imaging (MRI) is a clinically important medical imaging modality due to its exceptional soft-issue contrast. MRI scanners use the technique of nuclear magnetic resonance (NMR) to induce and detect a very weak radiofrequency signal that is a manifestation of nuclear magnetism. The term "nuclear magnetism" refers to weak magnetic properties that are exhibited by some materials as a consequence of the nuclear spin that is associated with their atomic nuclei. In particular, the proton, which is the nucleus of the hydrogen atom, possesses a nonzero nuclear spin and is an excellent source of NMR signals. The human body contains enormous numbers of hydrogen atoms, especially in water and lipid molecules.
The patient to be imaged must be placed in an environment in which several different magnetic fields can be simultaneously or sequentially applied to elicit the desired NMR signal. Every MRI scanner utilizes a strong static field magnet in conjunction with a sophisticated set of gradient coils and radiofrequency coils. The gradients and the radiofrequency components are switched on and off in a precisely timed pattern, or pulse sequence. Different pulse sequences are used to extract different types of data from the patient.
After scanning, MRI systems must provide a variety of mechanisms to create image contrast. If magnetic resonance images were otherwise restricted to water density, MRI would be considerably less useful, since most tissues would appear identical. Fortunately, many different MRI contrast mechanisms can be employed to distinguish between different tissues and disease processes.
The primary contrast mechanisms exploit the magnetization relaxation phenomena. The two types of relaxations are termed spin-lattice relaxation, characterized by a relaxation time T.sub.1, and spin-spin relaxation, characterized by a relaxation time T.sub.2.
Spin-lattice relaxation describes the rate of recovery of magnetization toward equilibrium after it has been disturbed by radiofrequency pulses. White matter has a shorter T.sub.1 than gray matter, so it produces a stronger signal. The stronger signals then shows up brighter in an image. Because the image highlights the parts with shorter T.sub.1, the image is "T.sub.1 -weighted."
Spin-spin relaxation describes the rate at which the NMR signal decays after it has been created. The signal is proportional to the transverse magnetization. White matter has a shorter T.sub.2 than gray matter, so it produces a weaker signal. Conversely, cerebrospinal fluid (CSF) has a long T.sub.2 and produces more signal. The stronger signals then shows up brighter in an image. Because the image highlights the parts with longer T.sub.2, the image is "T.sub.2 -weighted."
CSF creates many difficulties when obtaining heavily T.sub.2 -weighted images. For example, image pixels in close proximity with CSF are at risk of obscuring subtle contrast differences by inclusion of partial volumes of bright CSF signals. Further distortions are also introduced by the flow of CSF during scanning. Accordingly, it may be difficult to detect subtle lesions or disease processes.
Several pulse sequencing techniques have been proposed to suppress the magnetic resonance signal of CSF in conventional and rapid spin echo imaging. One of these techniques is called Fluid Attenuation by Inversion Recovery (FLAIR). The first implementation of this technique includes generating a nonselective 180.degree. pulse followed after a delay T.sub.delay by a complete multi-slice spin-echo sequence to produce a set of slices at different levels. The inversion time TI is equal to T.sub.delay for the first slice acquired and increased with successive slices. Sequence times are chosen so that the central slice has a TI appropriate for the null point of CSF (about 2200 ms at 1.0 T).
This approach is particularly useful with short echo times, since the slices are acquired in rapid succession over a short interval of time centered on the null point of the CSF magnetization. With longer echo times, the CSF signal is not completely nulled for the slices acquired at the start and finish of the multi-slice set, but, provided these were within approximately 500 ms of the null point, the CSF signal is still sufficiently reduced. In addition, the scans are insensitive to CSF flow distortions during the TI and TE periods since the CSF signal is suppressed. This technique provides an image that displays CSF as dark while maintaining strong T.sub.2 -weighing for the rest of the tissues.
However, because the T.sub.1 of fluids can range on the order of two and three seconds, the TI required to obtain an inversion is also long. This constitutes a significant disadvantage of the FLAIR technique, as the increased acquisition time required to accommodate the longer inversion times (1500 ms and higher) prohibitively extends the scan time. Even with the use of rapid spin echo techniques to shorten FLAIR scan times, the sequence can require up to 20 minutes in order to obtain anatomic coverage of a volume acquired in approximately 6 minutes by conventional rapid spin echo sequences. These long imaging times reduce patient compliance as well as expend precious medical resources.
Accordingly, persons skilled in the art have attempted to obtain the same results with shorter scan times. One approach has been to interleave all pulses for the different slices in one time period and to receive the late slice-selective echo trains for the slices in another period. However, a time period between the pulses and the echo trains where nothing occurs, or "dead time," still exists. This, of course, minimizes the efficiency of the procedure. Therefore, while the FLAIR technique is faster than the prior techniques, it is still slow.
An approach to overcome these difficulties have been proposed by C. H. Oh et al. In this approach, 180.degree. inversion pulses corresponding to different slices are interwoven with the conventional spin echo data acquisition sequence. However, using the conventional spin echo sequence as the basis for this procedure is problematic for several reasons. In conventional spin echo, the relaxation time period, or "TR," necessary for a slice to recover before being interrogated again is usually between 2000 and 2500 ms. However, executing FLAIR with such a short TR would result in very long times (20 to 30 minutes).
In addition, under the conventional spin echo technique, the MRI scanner cannot skip to other distant slices after exciting and scanning a slice. Accordingly, the scanned slices are distorted due to the concurrent CSF flow and/or cross-talk between slices.
It is the object of the present invention to provide a faster method for magnetic resonance imaging data acquisition that suppresses the CSF signal and an apparatus to carry out this method.